In order to test engines or to hold an airplane stationary immediately before take-off, it is known that, on airplanes fitted with conventional hydraulic brakes, some pilots have become accustomed to actuating the parking brake selector in order to hold the airplane stationary without it being necessary during such testing to keep applying pressure to the brake pedals.
Actuating the parking brake selector connects all of the brakes of the airplane to one or more of the hydraulic accumulators which exert pressure on the pistons of the brakes. The force generated by the pistons on the brake disks is then sufficient to hold the airplane stationary in spite of the thrust from its engines.
For an airplane fitted with brakes that are actuated electromechanically, the situation is more difficult.
A known braking architecture adapted to the particular case of an airplane having two main landing gear units, each fitted with two braked wheels is shown in FIG. 1 of the accompanying drawings.
Each of the wheels 1, 2, 3, and 4 is associated with a brake having four electromechanical actuators (referenced EMA in the figure, being numbered EMA1 to EMA4). The braking architecture has four electrical braking controllers 5, 6, 7, and 8 (referenced ECB in the figure) each controlling half of the actuators for both of the wheels in a corresponding one of the landing gear units. The electrical braking controllers receive braking orders from two braking and steering control units 9, 10 (referenced BSCU in the figure).
The airplane is also fitted with various generators of electricity (not shown) which are driven by the engines of the airplane, enabling first and second power supply buses PWR1 and PWR2 to be provided that are independent of each other, together with a third power supply bus PWREss. Finally, the airplane has an emergency electricity supply, generally comprising a source of direct current (DC) in the form of a battery or a set of batteries (Batt). For segregation purposes, no one controller EBC should be powered by both power supply buses PWR1 and PWR2. Likewise, no one controller EBC should be powered by one of the power supply buses PWR1 or PWR2 and by the DC source Batt. However it is acceptable for a given controller EBC to be powered by the power supply bus PWREss and by the DC source Batt.
This disposition therefore leads to using an architecture of the kind shown in FIG. 1, in which a first controller EBC (referenced 5) is powered by the first power supply bus PWR1, a second controller EBC (referenced 8) is powered by the second supply bus PWR2, and the other two controllers EBC (referenced 6 and 7) are powered by the third power supply bus PWREss and by the DC source Batt.
When the airplane engines are off, only the controllers EBC 6 and EBC 7 and the actuators EMA that are connected thereto can operate, being powered from the DC source, which is the only source that remains available.
Reference can be made to US patent document US-2001/045771A which illustrates such a braking architecture.
In conventional manner, that type of architecture is configured to present various braking modes (normal mode, alternate mode, emergency mode, automatic mode), during which the actuators apply a force to the stack of disks, where the force is established either as a function of signals coming from brake pedals actuated by the pilot, or as a function of a reference value for airplane deceleration, or as a test function reference value. These modes are referred to below as braking modes. They correspond to the nominal function of the brakes which is to absorb at least some of the kinetic energy of the airplane in order to slow it down.
That type of architecture is also configured to present a mode of operation referred to as parking mode during which the electromechanical actuators connected to the battery, i.e. the actuators EMA that are connected to the EBC controllers referenced 6 and 7, are controlled to develop a predetermined unit parking force. This mode is activated whenever the airplane is parked, and its function is to prevent the airplane from moving.
To this end, the architecture includes a selector 11 having two positions marked OFF and PARK. When the selector 11 is in the OFF position, the braking architecture is configured to operate in one of the braking modes, in particular in a normal braking mode during which all of the controllers EBC control the corresponding actuators EMA on the basis of braking reference signals generated by the braking control units BSCU. These reference signals are derived, amongst other things, from signals from pedals actuated by the pilot.
When the selector 11 is in the position PARK, the braking architecture is configured to operate in parking mode for which the two controllers EBC 6 and 7 connected to the battery assembly Batt are programmed to apply a predetermined unit parking force to the actuators EMA that they control.
The use of the actuators EMA that are associated with the controllers connected to the battery assembly, makes it possible to apply a parking force, even when the electricity generators of the airplane are not in operation. This disposition is useful, after the airplane has been moved in a parking area, for preventing it from moving any further, and without it being necessary to start the engines of the airplane.
The actuators EMA are fitted with a mechanical locking mechanism (not shown) which, after the parking force has been applied, serves to lock the actuators EMA in a position where they apply the parking force, thus making it possible to switch off the power supply to the actuators EMA and thus take a load off the battery assembly of the airplane.
By way of example, for an airplane of the Airbus A320 type fitted with brakes, each having four electromechanical actuators, the actuators that are operated in parking mode, i.e. half of the actuators of the airplane, deliver a unit parking force that amounts to substantially 33% of the maximum force they are capable of delivering (which corresponds to braking when take-off has been refused and the airplane is fully loaded), and that suffices to hold the airplane at maximum weight on a parking area having a slope of 3%.
It might therefore be thought, by analogy with airplanes in which the braking architecture is hydraulic, that the airplane could be held during engine run-up by switching to parking mode. However, given the force developed by the engines of the airplane while they are being run up, the unit force required of the actuators exceeds 60% of the maximum force they are capable of delivering.
So either the actuators need to be dimensioned so as to be capable of accommodating such a force, which would be completely unacceptable in terms of weight and power, or else known architectures of braking systems for airplanes fitted with brakes having electromechanical actuators cannot hold the airplane steady during engine run-up solely by making use of parking mode, and they therefore oblige the pilot to keep the brake pedals pressed down during engine run-up.